In recent years, a great attention is focused onto a solid polymer membrane fuel cell, as a household power supply and a power supply for a vehicle, which uses a solid polymer membrane as an electrolyte of a fuel cell to lower its operating temperature while providing a miniaturized and light weight structure.
A fuel cell body usually includes a fuel electrode supplied with fuel gas, and an air electrode supplied with air, with hydrogen contained in the fuel gas and oxygen contained in air being electrochemically reacted to generate electric power output. Unused exhaust fuel gas and unused exhaust air, which are not consumed in the fuel cell body for generation of electric power output, are introduced to a combustor and combusted therein to produce a reaction product, which is composed of steam and inactive nitrogen and is expelled as harmless exhaust.
A power generation capacity of the fuel cell body depends on a partial pressure of hydrogen supplied to the fuel electrode and a partial pressure of oxygen in air supplied to the air electrode at a flow rate depending on the partial pressure of hydrogen. If the power output is taken out from the fuel cell body at a rate greater than such a power generation capacity, not only the fuel cell body encounters a deterioration in a power generating performance but also catalysts of the fuel electrode and the air electrode become high temperature to be adversely affected, or an electrolyte membrane (proton permeable membrane) are adversely affected.
For this reason, when taking out the power output from the fuel cell body, operations are required to confirm whether the fuel electrode of the fuel cell body is fully filled with fuel gas and to control the fuel cell body in a way to produce controlled power output to prevent an overload exceeding the power generating capacity.
Japanese Patent Application Laid-Open Publication No. 2000-315516 discloses a control device for a fuel cell system. With such a control device, when reforming feed materials in a reformer to produce hydrogen rich reformed gas, operation is executed to estimate the amount of resulting hydrogen from the flow rates of feed materials supplied to the reformer and the reformer temperature, and the fuel cell body is operated to produce the power output in dependence on such an estimated value.